In case of an inorganic fiber paper produced by wet paper making, a predetermined strength may be imparted to an inorganic fiber paper having a high basis weight of not less than 100 g/m2 by using fine diameter fibers, for example, having an average fiber diameter of not more than 1 μm as the inorganic fibers, owing to the entanglement of the fine diameter inorganic fibers therein; and therefore, an inorganic fiber paper may be obtained from materials of only inorganic fibers, not using a binder (for example, Patent References 1 and 2). However, for an inorganic fiber paper having a low basis weight of less than 100 g/m2, even though fine diameter inorganic fibers, for example, having an average fiber diameter of not more than 1 μm are used similarly, the paper could hardly have a satisfactory paper strength only by the entanglement of the fine diameter inorganic fibers therein; and even when any auxiliary strength-enhancing measure of using thick fibers and thin fibers as mixed for the inorganic fibers is employed, use of a binder is inevitable.
Heretofore, the binder usable in an inorganic fiber paper having such a low basis weight of less than 100 g/m2 includes an organic fibers such as a synthetic resin emulsion, a thermo-fusible synthetic fibers, micro-fibrillated organic fibers, and an inorganic binder such as an inorganic oxide sol; and it may be used by internal addition (pre-addition to a sheet making stock material) or external addition (post-addition to sheet after wet sheet making) (for example, Patent References 3 to 8).
However, in case where an organic binder is used as the binder, there may occur problems during use of the inorganic fiber paper, in that the organic binder may undergo oxidation-reduction decomposition owing to the influence of chemical substances or voltage thereon, or may be decomposed or fired by heat, or may generate a volatile gas or may form a side product owing to such decomposition or firing. For example, in case where an inorganic fiber paper is used at a temperature of from 200 to 600° C., the organic binder therein is not completely fibered but remains therein with forming a small amount of a side product. In case where an organic binder containing carbon, oxygen and hydrogen is used, it may generate an organic substance such as aldehyde, ketone and carboxylic acid in incomplete combustion. In case where an organic binder further containing nitrogen in addition to carbon, oxygen and hydrogen is used, it may generate a smelling nitrogen compound such as amine compound and nitrile compound in incomplete combustion.
In case where an inorganic binder such as an inorganic oxide sol (inorganic oxide sol-based inorganic binder) is used as the binder and when the inorganic oxide sol-based inorganic binder is added as internal addition during wet sheet making, there is a problem in that the retention yield of the inorganic oxide sol-based inorganic binder during sheet making is low and therefore a large amount of the inorganic oxide sol-based inorganic binder must be used; and when the inorganic oxide sol-based inorganic binder is added as external addition, there is a problem in that it lowers the flexibility of the inorganic fiber paper and lowers the producibility, thereby increasing the production cost.
Accordingly, as a method of using an inorganic binder except the inorganic oxide sol-based inorganic binder, Example 8 in Patent Reference 9 proposes an inorganic fiber paper having a basis weight of 100 g/m2 and produced from a material comprised of 85% by mass of micro-glass fibers (average fiber diameter, about 2.7 μm) and, as an inorganic binder, 15% by mass of synthetic swelling fluoromica (average fiber diameter, about 6 μm), by wet sheet making.
Patent Reference 1: JP-A 59-71255
Patent Reference 2: JP-A 60-119073
Patent Reference 3: JP-A 10-317298
Patent Reference 4: JP-A 60-81399
Patent Reference 5: JP-A 62-207780
Patent Reference 6: JP-A 60-58221
Patent Reference 7: JP-A 5-311596
Patent Reference 8: JP-A 2-251214
Patent Reference 9: JP-A 9-87992